In recent years, as a transmission method of a radio image transmission device (hereinafter, a field pickup unit (FPU)) that wirelessly transmits video and voice picked up on site to a television station, OFDM scheme is adopted as a modulation scheme robust over multipath fading, and used for fixed and mobile radio relay (see Non Patent Literature 1).
A conventional antenna direction adjustment method has adopted a method that detects a reception electric field level by a detector at a reception side, searches for a direction in which the reception electric field level is the maximum, and adjusts the direction of an antenna. However, it has been difficult to detect accurately when the reception electric field is low, because a reception signal is buried in noise. More specifically, because in an early stage of antenna direction adjustment, the reception electric field is very low, even if an OFDM signal has arrived albeit only slightly, the reception signal buried in noise cannot be captured, and direction adjustment had to be conducted through a trial-and-error manner.
As a conventional antenna direction adjustment method in the OFDM scheme that has improved the above-mentioned drawbacks, one example is Patent Literature 1. This method will be briefly explained below using FIG. 6.
An OFDM signal that was transmitted from a transmission side and arrived at a reception device is received by a reception antenna 1 and frequency-transformed into a baseband signal by a reception high-frequency unit 2. The baseband signal is input into an A/D conversion circuit 3 and a reception sampling sequence x(t) (“t” is a sample number) is obtained. Then, a demodulated signal D is output to an external device after going through a path of a main line system composed of an FFT (Fast Fourier Transform) circuit 6, a demodulation unit 7, and the like to demodulate a transmitted information code.
In addition, the reception sampling sequence x(t) is input into a synchronization processing unit 8 that synchronizes clock timing and a carrier frequency with a transmission signal, and a synchronization signal SYNC is distributed to the whole OFDM reception unit as a signal to control reception timing.
Furthermore, the reception sampling sequence x(t) is connected with a reception electric field calculation unit 4 that calculates a reception electric field, together with the above-mentioned connection. A reception electric field signal R obtained by the reception electric field calculation unit 4 is connected with a direction adjustment signal generation unit 5, and the direction adjustment signal generation unit generates a direction adjustment signal C of the reception antenna 1.
Next, the configuration of the reception electric field calculation unit 4 will be described further in detail. FIG. 7 is a diagram showing the configuration of the reception electric field calculation unit 4. The reception sampling sequence x(t) from the A/D conversion circuit 3 is connected with an effective symbol delay unit 4-1, and output of a reception sample sequence x(t-τ) delayed by an effective symbol is connected with a complex multiplication unit 4-2. Another input terminal of the complex multiplication unit 4-2 is connected with a reception signal from the A/D conversion circuit 3. As described below, an output signal of the complex multiplication unit 4-2 is connected with an integration unit 4-3, and is output as a reception electric field signal R via an absolute value unit 4-4.
The reception electric field calculation unit 4 performs processing using correlativity of an OFDM guard interval signal. An OFDM signal including the guard interval signal will be explained using FIG. 8 before explaining operation of the reception electric field calculation unit 4.
The OFDM scheme is a method that digitally modulates, at a constant symbol period Ts, hundreds to thousands of carriers arranged at a constant frequency interval, and transmits them. For modulation to an OFDM signal, IFFT (Inverse Fast Fourier Transform) with the number of points τ (for example, τ=1024) is usually used. One symbol of a transmission signal transmitted from the transmission side is composed of a signal (A+a) with an effective symbol period Ta made up of an IFFT-modulated OFDM signal with τ points, and a guard interval signal a′ with Mg points in which a signal “a” in an Mg (for example, Mg=128) point period Tg at the end of the one symbol is copied to a guard period Tg before the effective symbol period Ta. In addition, about b and b′ parts of the next symbol, the situation is the same.
Based on the above-described knowledge, processing performed by the reception electric field calculation unit 4 in FIG. 4 will be explained using FIG. 7. A signal in FIG. 9(a) that was sampled by the A/D conversion circuit 3 and input into the reception electric field calculation unit 4 is delayed by the number of sampling times τ (for example, τ=1024) corresponding to the effective symbol period Ta as in FIG. 5(b) by the effective symbol delay unit 4-1. The signal delayed by the effective symbol period Ta and the signal before delay are complex multiplexed per sample point by the complex multiplication unit 4-2, and the following is calculated.C(τ)=x(t)×x(t−τ)  (1)
An example of a waveform of an I component among I/Q components of the complex multiplication signal is shown in FIG. 9(c).
Guard interval signals a and a′ and b and b′ (dashed lines in the figure) in FIG. 9(c) have correlativity and their correlation level is higher than other periods.
The correlation level indicates a small value when the reception electric field level is low, and conversely, when the correlation level is high, which means that the reception electric field level is high.
For each of the thus-obtained I component and Q component of the output signal C(t) from the complex multiplication unit 4-2, the integration unit 4-3 performs averaging processing to suppress a harassing component, and extracts an effective correlation component. In other words, when the amount of contained noise becomes large, it becomes difficult to obtain a correlation waveform from which correlation level difference between the guard interval period and the period other than that can be determined, but by performing integration (averaging processing), an integration result other than the guard interval period converges to zero because there is no correlation in the period other than the guard interval period. On the other hand, the guard interval period has correlativity, and by performing integration processing, a correlation level depending on a CN ratio can be obtained. In addition, the configuration of the integration unit 4-3 can be realized by moving averaging processing performed by an FIR filter or the like, that is, a low pass filter (hereinafter, LPF), or an LPF made up of an IIR filter. Regarding the time constant of the LPF, it is preferable that the time constant is within hundreds msec as a time constant capable of quickly following antenna direction adjustment control.
The output of the complex multiplication unit 4-2 is input into the absolute value unit 4-4, and absolute values of the I component and the Q component are calculated.
As a calculation method for the absolute values, calculating sum of squares of the I component and the Q component enables calculation of the reception electric field signal R proportional to reception power as shown in a formula (2).I2+Q2  (2)
Because reception devices usually adopt a method that controls the level of a reception signal largely variable depending on a reception condition to an approximately constant level by an automatic gain control (AGC) circuit, and then performs a variety of signal processing, the power of the signal x(t) input into the reception electric field calculation unit 4 is always maintained virtually constant. In the case of such control being performed, the reception electric field signal R from the reception electric field calculation unit 4 cannot be a value proportional to the reception electric field.
FIG. 10 is a diagram showing a characteristic of the reception electric field signal R relative to the CN ratio. Especially, when the CN ratio becomes high, i.e., the reception electric field level becomes high, the level of the reception electric field signal R saturates at a certain constant value. If the level of the reception electric field signal R is in a saturation region, however, it may be determined that antenna direction adjustment is completed because synchronization is also established and the reception electric field higher than a prescribed level is obtained.
Thus, in the early stage of reception antenna direction adjustment, even if the CN ratio of a received OFDM signal is about 0 dB or below, i.e., even if the reception electric field is about −97 dBm or below, presence of the OFDM signal can be adequately detected, and the direction in which the level of the received OFDM signal is the maximum can be searched for while the direction of the reception antenna is being changed.
It has been difficult for the conventional antenna direction adjustment method that detects the reception electric field level by the detector at the reception side and searches for the direction in which the reception electric field level is the maximum, to perform accurate detection because the reception signal is buried in noise when the reception electric field is low. Further, as a modulation scheme of sub-carriers in the same OFDM, there is a scheme including a pilot carrier like a QAM (Quadrature Amplitude Modulation) scheme and a scheme only including a data carrier like a differential scheme. In an antenna direction adjustment method when receiving an OFDM signal for which any of those schemes was used, it has been difficult to discriminate whether the used scheme is the QAM scheme or the differential scheme, and to apply an optimal direction adjustment signal level.